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Subject: Re: Pipes truncating data in cygwin from main and cygwin-3_4-branch
Date: Tue, 15 Aug 2023 18:09:50 +0900
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To: Takashi Yano <takashi DOT yano AT nifty DOT ne DOT jp>
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> On Aug 15, 2023, at 15:42, Takashi Yano <takashi DOT yano AT nifty DOT ne DOT jp> =
wrote:
>=20
>>=20
>>>=20
>>> New pipe implementation since cygwin 3.4.x provides the pipes
>>> more similar to pipe in command prompt for non-cygwin apps.
>>=20
>> What are the differences between these pipes? What changed?
>=20
> Many changes. But, the change which triggers this behaviour
> is setting FILE_SYNCHRONOUS_IO_NONALERT create option.
>=20
> With this option, seekg() of Microsoft library gets success
> for pipes, despite it should not I suppose.

On what create is this option specified? I have the seekg() test because =
on some systems you can seek on stdin when it is being redirected to =
read from a file, or so I read somewhere. Perhaps I should always buffer =
stdin on Windows to avoid this problem.

I will put some tracing in `test-pipe.c++` to see whether it is =
currently buffering or not.

>=20
> In Linux, man page states that fseek() on pipes will fail.
>=20
>>> Since your test case is compiled with cl.exe, it is non-cygwin
>>> apps.
>>=20
>> When the failure first appeared the bash shell included with Git for =
Windows was being used to run the pipe (`bash -c "cat foo | bar=E2=80=9D`)=
. The shell was started by ctest which had been run from PowerShell. Is =
this cygwin or non-cygwin?
>=20
> It depends on "bar". If bar is compiled with cl.exe or gcc/g++ in
> MSYS2 mingw environment, it is non-cygwin app.
>=20
>> in =
https://github.com/git-for-windows/git/issues/4464#issuecomment-1671137446=
 the author provided minimal adaption to the reproducer to compile it =
with g++ and reproduced the failure on Cygwin. It looks like the failure =
happens for both cygwin and non-cygwin. It is probably related to the =
new pipe implementation you mentioned.
>=20
> Is there any failure case where the pipe reader is cygwin (or MSYS2)
> binary?

We=E2=80=99ve only tested non-Cygwin consumers. When the author of the =
above GitHub issue comment was testing on Cygwin he compiled the =
consumer with mingw-w64 g++.

Regards

    -Mark

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